Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase encoded by two isoforms, GSK-3α and GSK-3β with molecular weights of 51 and 47 kDa, respectively. These share 97% sequence similarity in their kinase catalytic domains. The GSK-3α isoform has an extended glycine-rich N-terminal tail. A minor splice variant of GSK-3β has been identified (expressed at ˜15% of total) with a 13 amino acid insert within the kinase domain. This variant had a reduced activity towards tau. GSK-3 is highly conserved throughout evolution, and found in all mammalians thus far with high homology in the kinase domain. Both isoforms are ubiquitously expressed in mammalian tissues, including the brain. Pharmacological GSK-3 inhibitors are not able to selectively inhibit one of the isoforms.
GSK-3β plays an important role in the control of metabolism, differentiation and survival. It was initially identified as an enzyme able to phosphorylate and hence inhibit glycogen synthase. Subsequently, it was recognised that GSK-3β was identical to tau protein kinase 1 (TPK1), an enzyme that phosphorylates tau protein in epitopes that are also found to be hyperphosphorylated in Alzheimer's disease and in several taupathies.
Interestingly, protein kinase B (AKT) phosphorylation of GSK-3β results in a loss of kinase activity, and it has been proposed that this inhibition may mediate some of the effects of neurotrophic factors. Moreover, phosphorylation of β-catenin (a protein involved in cell survival) by GSK-3β, results in its degradation by an ubiquitinilation dependent proteasome pathway.
Therefore it appears that inhibition of GSK-3β activity may result in neurotrophic activity. There is evidence that lithium, an uncompetitive inhibitor of GSK-3β, enhances neuritogenesis in some models and can also increase neuronal survival, through the induction of survival factors such as Bcl-2 and the inhibition of the expression of proapoptotic factors such as P53 and Bax.
Further studies have shown that β-amyloid increases GSK-3β activity and tau protein phosphorylation. Moreover, this hyperphosphorylation as well as the neurotoxic effects of β-amyloid are blocked by lithium chloride and by a GSK-3β antisense mRNA. These observations taken together suggest that GSK-3β may be the link between the two major pathological processes in Alzheimer's disease: abnormal APP (Amyloid Precursor Protein) processing and tau protein hyperphosphorylation.
These experimental observations indicate that compounds which modulate the GSK-3β activity may find application in the treatment of the neuropathological consequences and the cognitive and attention deficits associated with Alzheimer's disease, as well as other acute and chronic neurodegenerative diseases. These include, but are not limited to: Parkinson's disease, tauopathies (e.g. frontotemporoparietal dementia, corticobasal degeneration, Pick's disease, progressive supranuclear palsy) and other dementia including vascular dementia; acute stroke and others traumatic injuries; cerebrovascular accidents (e.g. age related macular degeneration); brain and spinal cord trauma; peripheral neuropathies; retinopathies and glaucoma.
GSK-3β may also have utility in the treatment of other diseases such as: Non-insulin dependent diabetes and obesity; osteoporosis; manic depressive illness; schizophrenia; alopecia; cancers such as breast cancer, non-small cell lung carcinoma, thyroid cancer, T or B-cell leukemia and several virus-induced tumors.
A review on GSK-3, its functions, its therapeutic potential and its possible inhibitors is given in “Glykogen Synthase Kinase 3 (GDK-3) and its inhibitors: Drug Discovery and Developments” by A. Martinez et al. (editors), John Wiley and Sons, 2006.
B. Barth et al. (Antiviral Chemistry & Chemotherapy 7 (6), 1996, 300-312) describe 6-alkyl substituted pyridazino[3,4-b][1,5]benzoxazepin-5-ones which are useful as inhibitors of HIV-1 reverse transcriptase. They also describe several pyridazino[3,4-b][1,5]benzoxazepin-5(6H)-ones being unsubstituted at the nitrogen as intermediates, namely pyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloropyridazinobenzo[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloro-8-trifluoromethylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloro-8-methylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloro-9-methylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloro-8-methoxypyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one and 3-chloro-8,10-dimethylpyridazinobenzo[3,4-b][1,5]benzoxazepin-5(6H)-one.
G. Heinisch et al. (Arch. Pharm. Pharm. Med. Chem. 2000, 333, 231-240) describe pyridazinobenzo[3,4-b][1,5]benzoxazepin-5(6H)-ones being unsubstituted at the nitrogen as intermediates in the synthesis of the corresponding N-alkyl derivatives, namely 3-chloropyridazinobenzo[3,4-b]-[1,5]benzoxazepin-5(6H)-one, 3,8-dichloropyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one, 3-chloro-8-methylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one and 3-chloro-9-methylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one.
I. Ott et al. (J. Med. Chem. 2004, 47, 4627-4630) describe 6-alkyl substituted pyridazinobenzo[3,4-b][1,5]benzoxazepin-5-ones which are useful as Multidrug-Resistance Modulating agents in tumor therapy. They also describe several pyridazinobenzo[3,4-b][1,5]benzoxazepin-5(6H)-ones being unsubstituted at the nitrogen as intermediates, e.g. 3-chloro-9-trifluoromethylpyridazino[3,4-b][1,5]benzoxazepin-5(6H)-one.
G. Heinisch et al. (Arch. Pharm. Pharm. Med. Chem. 1997, 330, S. 29-34 and Heterocycles 1997, 45, 673-682) describe inter alia 3-chloro-8-nitro-11-propyl-pyridazino[3,4-b][1,5]benzodiazipin-5-one.